1. Field of the Invention
The present invention relates to an apparatus for storage and reconstruction of a radiation image, and more particularly to an apparatus for storage of a two-dimensional radiation image using a phosphor plate and for reconstruction of a stored radiation image.
2. Description of the Prior Art
A method for observing a radiation image of an object by using a radiation such as X-ray, .alpha.-ray, .beta.-ray and .gamma.-ray is commonly used in the field of a medical diagnosis or for non-destructive inspection.
In general, a silver halide-based film is utilized as a radiation image storage medium. Nevertheless, a new apparatus is being developed in order to conserve silver and to digitize an image signal.
A typical prior art method is as follows;
In storage stage, an X-ray emitted from an X-ray source is irradiated to an object such as a human body, and a transmitted radiation image is stored in an image storage phosphor plate. The image storage phosphor plate is made of a silver-activated Barium fluoro halide. Entering the X-ray into the image storage phosphor plate, active are formed in the phosphor plate according to an X-ray image.
In a reconstruction stage, a laser beam from a He-Ne laser (wavelength: 633 nm) or a He-Cd laser (wavelength: 325 nm) is deflected by a deflector, and the phosphor plate is scanned by the deflected laser beam. The phosphor plate is simultaneously moved in a direction orthogonal to the scanning direction. Consequently, a flourescent light, the intensity of which corresponds to the irradiated X-ray, is emitted from the phosphor plate. The flourescent light is collected by a light collecting sheet which is made of an acrylic resin, which is then fed to a photo multiplier. A current signal obtained from the photo multiplier is digitized in a data processing device, and, after a desired image treatment is carried out, a radiation image is displayed by an image display device. Several image processing methods are well known. One of the typical methods is a so-called pulse image mode processing, in which a plurality of subtraction images are formed synchronously with the pulsed X-ray exposure, with one image then being formed by adding each substraction image. Other methods such as a so-called continuous mode processing and a time interval difference mode processing are also well known.
However, these prior art methods have several disadvantages.
A fluorescent light emitted from a radiation image storage medium by the scanning laser beam has both a light component which is collected by the light collecting sheet and a light component wich is transmitted in the medium and is not collected by the light collecting sheet. In a silver halide-based radiation image storage medium, the light component which is transmitted and is not collected by the light collecting sheet is more than fifty percent of the total light. Therefore, the amount of light collected by the light collecting means is small, and thus the output signal from the photo multiplier is also reduced, and the quality of the reconstructed radiation image is degraded. In order to avoid this problem, the intensity of the radiation which is irradiated to the object must be increased.
However, increase in radiation intensity has harmful effect upon the human body, and likewise is a problem for the system itself.
In the prior art, it is also difficult to reduce the necessary time period for reconstruction of the radiation image, because a mechanical scanning means such as the deflector and a mechanism for moving the radiation image storage medium are used when the light excitation is carried out against the radiation image storage medium to reconstruct the radiation image.
Furthermore, for the same reason, an instantaneous image processing is difficult, because the radiation image storage medium must be set up to the attached reconstruction apparatus after the radiation image is stored in the medium.